The invention is directed to a method for filtering signals, whereby signal amplitudes can be allocated to a quantized, two-dimensional reference plane, and whereby a filtered signal can be added to an unfiltered input signal for amplification of a useful signal part after constant parts as well as high-frequency parts have been filtered out.
Over and above this, the invention is directed to an apparatus for filtering signals, whereby signal amplitudes can be allocated to a quantized, two-dimensional reference plane, and whereby a separating filter that branches the input signal as well as a frequency-dependent filter arranged in a filter branch are provided.
Such methods and apparatus for two-dimensional filtering of signals to reduce noise are particularly employed in electronic image processing in order to improve the quality of images.
Various methods for two-dimensional filtering are already known from the prior art. EP-A-0 227 848 discloses a method with which isolated, black locations in an image can be eliminated, namely small regions are viewed for this purpose and a determination is made whether a plurality of black points is situated therein whose percentage is higher or lower than a specific, prescribed percentage with reference to the total number of points in this region. When the percentage is higher than the aforementioned limit value; then the black points are left, otherwise, they are converted into white points. This serves the purpose of eliminating black, disturbing points on scanned originals.
DE-A-34 23 112 is concerned with the problem of eliminating weak background noise. In order to reduce this, the signal is first split into two signal paths: the one merely delays the input signal so that it is isochronic to the output signal on the other branch, to which it is subsequently added and as a result whereof the output signal of the arrangement is formed. The following signal editing is carried out in the other branch: the signal initially passes through a band-pass filter that is matched as well as possible to the power spectrum of the image signal. Among other things, this band-pass filter eliminates all low-frequency and constant parts. The output signal of this band-pass filter is subsequently supplied to a threshold switch that allows all the signal parts to pass that are higher than a defined threshold which is dynamically acquired from the image signal, but which blocks all of the signal parts that lie below this threshold. Since these latter signal parts are usually a matter of noise, this leads to a suppression of noise, since such signals are extremely rare in a real image signal. The resulting output signal is then multiplied by a controllable factor and is added to the afore-mentioned input signal that has been looped through with a delay. The reason for the loop-through of the unedited input signal is that, if the above-presented signal processing circuit should happen to lead to excessive image falsification, the influences of these can be limited.
U.S. Pat. No. 4,573,070 is directed to a two-dimensional image filter whose properties are constantly dynamically matched to the respective properties of the image signal that is just present. Since its functioning, however, departs rather substantially from the method of the application, this shall not be discussed in greater detail here.
EP-B-0 051 068 discloses another filter means. It turns out, however, that the known filtering methods and filter devices do not supply satisfactory results, particularly when the amplitude of the useful signal is not clearly higher than the amplitude of superimposed disturbances. This results therein that signals having a low signal-to-noise ratio can only be inadequately filtered. When, given image signals, only weakly demarcated, comparatively regular textures are present on a surface, then it is already not possible to undertake an adequately sharp working-out of the contours limiting the texture elements even given comparatively slight disturbances.